1. Field of the Invention
The present invention relates to a CDMA/TDD mobile communication system and method applying a same band transmission/reception system for assigning time slots at the same radio frequency band to communicate over a reverse link and a forward link alternately.
2. Description of the Invention
Conventionally, as a mobile communication system using a CDMA (Code Division Multiple Access) system, a W-CDMA (Wideband CDMA) system using a DS (Direct Sequence) system is known. The W-CDMA system uses a FDD (Frequency Division Duplex) system as a duplex system.
As a duplex system, a TDD (Time Division Duplex) system is known other than the FDD. The TDD system uses a same band transmission/reception system for assigning time slots at the same radio frequency band to communicate a reverse link and a forward link alternately, which is also called Ping-Pong system.
In addition, a multiple access system is a line connection system over which a plurality of stations concurrently communicate at the same frequency band. The CDMA system employs a technique for performing a multiple access by a spread spectrum communication in which information signals are spread by a spreading code to transmit over a spread band. In the DS system, information signals are multiplied by a spreading code in spreading.
In the DS-CDMA system, since a plurality of communication links share the same frequency band, there is a problem to control each communication wave level at a reception to be equal (near-far problem). In other words, the conquest over this problem is necessary to achieve the CDMA communication system.
The near-far problem is severer at a reception of a base station for concurrently receiving radio signals transmitted from a plurality of mobile stations (mobile radio terminal devices) each locating at a different place. Therefore, it is mandatory at the mobile station propagation path condition.
In the TDD system, propagation path conditions such as fading correlate when intervals of forward and reverse links are short enough because the same frequency band is used for the forward and reverse links. It is thereby possible to perform a transmission power control by open-loop control.
Some of such CDMA mobile communication system comprises a TDD configuration for dividing a communication frame into a plurality of time slots to assign each of traffic channel and control channel to a time slot to communicate over a plurality of links.
FIG. 1 illustrates a frame diagram in a conventional CDMA/TDD mobile communication system.
FIG. 1 illustrates an example where one frame 1 is divided into eight time slots 0 to 7, time slots 0 to 3 are assigned to a forward link and time slots 4 to 7 are assigned to a reverse link.
In forward link 2 toward a mobile station (not shown) from a base station (not shown), common control channel 3 such as synchronization control channel, a dedicated control channel and user information channel 4 that are transmitted between the base station and a dedicated mobile station are multiplexed. In reverse link 5 toward the base station from the mobile station, the dedicated control channel and user information channel 6 are multiplexed.
The mobile station starts receiving when it is turned on, and acquires synchronization with the base station by detecting the synchronization control channel of common control channel 3 in forward link 2. Then the mobile station performs a line connection via the dedicated control channel to start communicating with user information channel 4. At this point, the mobile station measures reception quality of an assigned forward link time slot, and based on the measurement result, performs a transmission power control of an assigned reverse link time slot.
However, in the conventional CDMA/TDD mobile communication system described above, when an assignment of time slots to forward link and reverse link is changed corresponding to information volume of the forward and reverse links, a configuration of the synchronization control channel becomes irregular. It thereby takes a longer time for a mobile station to acquire synchronization with a base station when it is turned on.
In addition, in each communication link, since an interval of the assigned forward link time slot and the assigned reverse link time slot is long, the propagation path conditions of the forward and reverse links do not correlate so much, thereby decreasing an effect of an open-loop controlled transmission power control.
It is an object of the present invention to provide a CDMA/TDD mobile communication system and method for enabling a synchronization acquisition time with a base station at a mobile station to be shortened, and enabling open-loop controlled transmission power control to function effectively when an assignment of time slots to forward and reverse links is changed corresponding to an information volume in the case where the information volumes of the forward and reverse links are asymmetry.
The object is achieved by a CDMA/TDD mobile communication system comprising a base station apparatus having a control section for assigning one or more fixed forward link time slots to a plurality of time slots divided at a communication frame at a predetermined interval, while assigning forward link time slots and reverse link time slots to the time slots except for the fixed forward link time slots by allocating the time slots corresponding to a ratio of a total information volume of a forward link to a total information volume of a reverse link in the system, in order to transmit a control channel signal including a synchronization control channel signal using the fixed forward link time slots, and to transmit a traffic channel signal of the forward link and a traffic channel signal of the reverse link respectively using the forward link time slots and the reverse link rime slots that are allocated according to the number of time slots respectively required corresponding to the respective information volume.